Hydraulically lockable lifting device

ABSTRACT

A hydraulically lockable lifting device particularly adapted for height adjustment of tables comprising a gas spring in which a piston mounted in a piston rod and acting against compressed gas travels in a tube which is sealed with respect to the outside, wherein both sides of the piston are connected by an openable passage that connects chambers filled with an incompressible medium and a pump for pumping the incompressible medium from one chamber into the other.

[111 3,825,244 [451 July 23, 1974 Hickinbotham [54] HYDRAULICALLY LOCKABLE LIFTING 3,712,603 1 1973 DEVICE Fritz Bauer, Nurnberg, Germany [75] Inventor:

Primary Examiner-James B. Marbert- Assigneei i" Fedel'ungstechnik GmbH, Attorney, Agent, or FirmBrowdy and Neimark Altdorf b. Nurnberg, Germany Dec. 29, 1972 [22] Filed:

[2i] Appl. No.: 319,751

ABSTRACT Foreign Application Priority Data Dec. 28, l97l A hydraulically lockable lifting device particularly Germany...........................

. adapted for height ad ustment ofv tables comprising a "gas spring in which a piston mounted in a piston rod and acting against compressed gas travels in a tube 6 .1 .17 "1 l6 7 n 9 m m mh a m& L l m d s mm U IF l. 11] 2 l8 5 55 rL rtl [56] References Cited UNITED STATES PATENTS 3,444,830 5/1969 Doetsch 267/64 R 9 Claims, 6 Drawing Figures lib ll v,

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"a? n v .lllllllllihll iuaiiiwvi i nvvvui svus 1 HYDRAULICALLY LOCKABLE LIFTING DEVICE SPECIFICATION I mounted on a piston rod and acting against compressed gas travels in a tube which is sealed with respect to the outside, wherein both sides of the piston are connected by an openable connecting passage that connects chambers filled with an incompressible medium.

In lifting devices of this kind which are presently known, the compressed gas charge serves to lift the top of a table or the seat of a chair when the hydraulic lock is removed. To lower the tabletop or the seat of the chair, the table top or chair seat mustbe depressed as the hydraulic lock is simultaneously removed, so that the piston rod, togetherwith the piston, is pressed into the gas chamber, compressing the gas. Difficulties arise in this connection, especially in the case of tables, as

the force with which the table top is pressed downward usually cannot be applied coaxially to the lifting device, so that the lifting device, which'as a rule is mounted in an external guide tube, is tilted and therefore jams.

In addition, lifting devices are known such as are used in barber chairs, in which a unilaterally or bilaterally pressurized hydraulic piston is subjected to pressure from hydraulic fluid by a pump, said fluid being drawn from and returned to an expansion chamber or reservoir. Such devices are extremely bulky and therefore are not well suited for tables, in particular.

The purpose of the present invention is therefore to construct a hydraulically lockable lifting device of the type described above, in such fashion that a table can be lowered very simply against the gas pressure of the gas spring, without the danger of tilting and hence jamming of the lifting device as the table is lowered.

This problem is solved according to the invention by providing a pump to drive the' incompressible medium from one chamber into the other. While the lifting of the table or table top by the gas spring is accomplished through the expansion of the compressed gas contained in it, the lowering of the table is accomplished by pumping the incompressible medium (which serves to lock the gas spring) fromone chamber into the other, i.e., the piston and piston rod of the gas spring are, so

to speak, pumped back into the gas spring against the.

pressure of the gas.

This means that the force required to lower the table is directed absolutely centrally with respect to the lifting device, and the danger of tilting the lifting device therefore no longer exists. Furthermore, this makes it possible to give the lifting device any desired (for example, square) cross section, so that the table top is prevented from turning with respect to the foot, without additional roller guides on theoutside of the hous- Ing. I

According to a feature of the invention, if the total volume of all the chambers filled with the incompressible medium is constant, the additional provision of an expansion chamber or reservoir is not required.

Since the table is lifted by the gas spring and the pump is used only to lower the table, the pump is made so thatit operates in one direction only, according to an advantageous featureof the inventionQAbsolutely constant volume conditions can be obtained simply in 2 the pump by making the latter a plunger-piston pump.

According to a further advantageous feature of the invention, the chamber for the incompressible medium, which is located on the side of the piston charged with compressed gas, is mounted in a separate tube, in which an auxiliary piston is mounted as a sealing device to shut off the gas chamber from this chamber, said auxiliary piston resting againstthe piston or fastened to it. This simultaneouslyinsures that the gas chamber is always sealed off from this chamber and that absolutely constant volume conditions always prevail in the entire space filled with the incompressible medium. This auxiliary piston is advantageously provided with a cavity which communicates with the gas chamber, thereby enlarging the total 'gas volume, also leading to an improvement of the spring characteristics. The auxiliary piston is advantageously longer than the maximum travel of the piston.

A particularly simple structural design and an especially compact form are achieved by mounting the pump coaxially with the gas spring and with a valve arrangement in acommon housing. An extremely simple operability is achieved by mounting an operatingpin for the valve and a pump piston rod for the pump so that one is inside the other, and bringing them out together from the housing; the operating pin and pump piston rod can even be made in one piece.

It is highly advantageous if the plunger-piston pump consists in a pump piston which is guided in a working chamber by a pump piston rod and has check valves, so that each of the two parts of the working chamber separated by the pump piston is connected with a chamber. According to an especially advantageous embodiment of the invention, the pump serves simultaneously as a valve, with operating pins being advantageously provided for forcible opening of the check valves with the pump piston at one terminus of its travel. By combining the pump and valve in this fashion, the lifting device according to the invention is considerable simplified and made sturdier at the same time.

Further advantages and features will be seen from the description of sample embodiments as shown in the drawing: 1

FIG. 1 is a lengthwise cross-sectional view through a lifting device according to the invention;

FIG. 2 is a cross sectional view through a further embodiment of a lifting device; 7

FIG. 3 is a partial lengthwise cross-sectional view. through a further embodiment of the invention;

FIG. 4 is a lifting device according toFIGS. l to 3, installed in a height-adjustable table;

FIG. 5.shows a further embodiment of a lifting device according to the invention, in lengthwise cross section, and

FIG. 6 is a partial cross sectional view device according to FIG. 5.

The device shown in the drawing has a housing consisting of a steel tube 11, sealed at both ends by plugs 12 and 13 which fit gasor liquid-tight against the inner walls of the steel tube. By means of plug 13 shown at the bottom of the drawing, piston rod 14 is mounted concentrically inside housing 11. A seal 16, provided with several sealing lips and resting against piston rod 14, is mounted in groove 15 on plug 13, so that the inner chamber of housing 11 is gasor liquid-tight with of the lifting respect to the outside even at the point where piston rod 14 passes through plug 13. A cylindrical piston 17 is mounted on the end of the piston rod 14 which is inside the housing (by means of rivets, for example), the piston being axially displaceable together with piston rod 14 in steel tube 18 mounted in housing 11. Piston 17 has a ring seal 19 on its circumference, so that it is mounted gasor liquid-tight in steel tube 18.

A passage 20 which completely encircles steel tube 18 is provided between steel tube 18 and the inner wall of housing 11. At the end which is shown at the bottom of the drawing, ie in the vicinity of plug 13, steel tube 18 is mounted in centering ring 21, whose internal diameter is larger than the diameter of piston rod 14. Centering ring 21 has one or more radial bores 22 connecting passage 20 with chamber 23 formed by piston 17, piston rod 14 and the corresponding portion of steel tube 18. In steel tube 18 there is a gas chamber 24 on the side of chamber 23 that faces away from piston 17, this chamber being filled with compressed gas. The gas chamber is bounded on the inside by a tube 25, mounted concentrically in steel tube 18, the tube 25 being able to travel in the direction of plug 13 for a distance which corresponds to the maximum travel of piston rod '14, together with piston 17 into housing 11. A lengthwise cylindrical auxiliary piston 26 is mounted in tube 25, the piston having one end facing the gaschamber side of piston 17. In the vicinity of the other end, auxiliary piston 26 has a ring seal 27 mounted in an annular groove, so that the auxiliary piston rests gasor liquid-tight against the inner wall of tube 25. The length of auxiliary piston 26 is such that it continues to travel smoothly in tube 25 when piston 17 strikes centering ring 21 and auxiliary piston 26 simultaneously rests on piston 17 with ring seal 27. Auxiliary piston 26 is therefore longer than the maximum travel of piston 17 and piston rod 14 in housing 11.

Auxiliary piston 26 is essentially hollow inside. This cavity 28 is connected to gas chamber 24 by one or more bores 29 in the wall of auxiliary piston 26. The total gas chamber is therefore formed by the annular space between steel tube 18 and tube 25 as well as the outer wall of auxiliary piston 26, in addition to cavity 28 in auxiliary piston 26. This additional cavity 28 therefore increases the total volume of the gas chamber, thus leading to an improvement of the spring characteristics.

Steel tube 18 and tube 25 that fits into it to form gas chamber 24 are mounted with their upper (as shown in the drawing) ends gasor liquid-tight against partition 30, on the other side of which there is a pump housing 31 which will be described in greater detail below. Pump housing 31 also is mounted radially and axially fixed against upper plug 12. The outside diameter of pump housing 31 and the outside diameter of partition 30 are each smaller than the inside diameter of partition 30 are each smaller than the inside diameter of housing 11, so that passage 20 extends over the length of pump housing 31. In order that the part of the pump housing 31 that rests against partition 30 and partition 30. itself cannot make any radial movements, both of them have small projections 33 and 34 in this area that are distributed over the circumference and rest against the inner wall of the housing. Another chamber 35 is formed by partition 30, auxiliary piston 26 and the inner wall of tube 25.

Aligned cylindrical recesses 36,37 are provided in the surfaces of partition 30 and pump housing 31 that face each other. These recesses together form a cylindrical cavity. At least one radial bore which connects this cavity with passage 20 terminates in this cavity. A valve lifter 39 is mounted in cavity 36,37, coaxial to the latter and to the housing. Said valve lifter consists of a lengthwise cylindrical pin and annular groove 40. Valve lifter 39 extends through bore 41, which has a larger diameter than the valve lifter, into chamber 35. The valve lifter is sealed off by lip seals 42 and 43 located in the upper part of recess 37 and the lower part of recess 38. Between lip seals 42,43 there is a roughly cylindrical spacing sleeve 44, which has openings 45 to connect annular groove with bore 38.

Annular groove 40 is so long and deep that when valve lifter 39 is depressed, bore 41 is connected to radial bore 38 and/or openings to bypass lip seal 43.

A cylindrical chamber 46 is mounted inside tube 25 (i.e., in chamber 35) on the side of partition 30 that faces chamber 35, said chamber containing a small tensioned compression spring 47, said spring holding the valve lifter in its resting position (the upper position as shown in the drawing) by means of a pressure plate 48 attached to the valve lifter. In this position, there is no connection between the bore (and hence chamber 35) and radial bore 38 in pump housing 31 and hence passage 20.

Above the upper resting position of the pressure plate 48 (as shown in the drawing), a lip seal 49 is mounted axially fixed on the inner wall of chamber 46, said seal resting liquid-tight against valve lifter 39, so that the total volume of chamber 35 remains constant when pressure plate 48 and/or valve lifter 39 is pushed into chamber 46.

The total volume of chamber 35 also includes the space above lip seal 49 in chamber 46, since this space is connected with chamber 35 by means of fluid openings 50 in the wall chamber 46.

At the end opposite pressure plate 48, valve lifter 39 passes through a seal and runs through lip seal 42 into cylindrical cavity 51 in pump housing 31.

Above cylindrical cavity 51 pump housing 31 has a cylindrical working chamber 52 which extends up to upper plug 12. Pump piston rod 53 is mounted axially displaceably in plug 12 and cylindrical cavity 51, coaxially with housing 11 and working chamber 52, said rod being sealed off by ring seals 54,55 from plug 12 and the side wall of cavity 51. In working chamber 52, a pump piston 56 is mounted on pump piston rod 53, said piston being made liquid-tight against the side wall of working chamber 52 by means of seal 57. Pump piston 56 has several axial bores 58 that are provided on their upper sides with check valves 59. Working chamber 52 is connected to passage 20 in its upper area, i.e., in the area of plug 12, by one or more bores 60.

Pump piston rod 53 is made in the form of a hollow rod in which an operating pin 61 is fitted, said pin having its inner end resting freely against valve lifter 39.

Pump housing 31 has a concentric bore 62 that communicates with annular passage 63 in partition 30 and bore 64 that extends from the partition to chamber 35, so that the lower part of working chamber 52 communicates with chamber 35.

Gas chamber 24 is charged with compressed gas, for example, through a bore that is not shown but which passes through piston rod 14 and is sealed after the charging process is complete. Filling with hydraulic fluid is accomplished through a filler valve that is appropriately located but is also not shown.

If a table fitted with a lifting device has but one supporting column, it is advantageous to give housing 11 a cross section which is square (for example) rather than circular, to keep the table top from rotating with respect to its foot. FIG. 2 shows a section through a lifting device according to FIG. 1, in which housing 11' has a square cross section. It is primarily only plugs 12,13 and projections 33 and 34 that need to be adapted. Passage 20 necessarily assumes a somewhat different shape as a result, but otherwise the entire construction of the lifting device is identical to that shown in FIG. 1.

FIG. 3 is a further change in a sample embodiment of the invention, which differs from those shown in FIG. 1 and FIG. 2 only in thatpump piston rod 53 and operating pin 61 are combined to form pump piston rod 53 which also'serves simultaneously to operate valve lifter 39.

As can be seen from FIG. 4, the lifting device according to FIGS. 1, 2 or 3 is mounted in a guide tube 65, in this case with interposition of a plastic guide bushing 66. Guide tube 65 is fastened to foot 67 of a table.

The lower face end of guide tube 65 is provided with base 68 to which the free end of the piston rod 14 is axially fixed. The base consists for example of a beaded edge turned inward. This grips relatively deeply into the annular groove of an elastically flexible disc-shaped body 69, made of rubber for example. This body 65 is mounted between two metal discs 70 and, together with said discs, is traversed by the studlike free end 71 of piston rod 14, mounted axially fixed in the elastically flexible body 69 by a Seeger circlip 72.

The upper end of housing 11 or 11 is fastened by screws to the underside of a table top 75 by-means of a hollow boss in the form of a truncated cone 73 and a ring flange 74.

An operating. handle 76 is swivel mounted around an axis 77 in hollow boss 73 in the form of a truncated cone. The free end of this operating handle acts on the operating pin 61 and/or the pump piston rod 53 or the pump piston rod 53'.

The end of the hollow boss 73 that is opposite ring flange 74 is made cylindrical for a short part of its length. The internal diameter of this cylindrical part 78 roughly corresponds to the outside diameter of housing 11, whose upper face can rest against one connecting surface 79 of cylindrical section 78. The wall of this part is slotted and the slot is bounded externally by two flange clamps 80, which can be connected by a clamping screw 81. This results in an effective clamp connection by which the housing 11 or 11' of the lifting device is frictionally connected with table top 75 via hollow boss 73. In a lifting device according to FIG. 4, cylindrical part 73 is given a square cross section in accordance with the altered cross section of housing 11'. Guide tube 65 and guide housing 66 can be made long enough to strike cylindrical section 78 shortly before piston 17' hits tube 25 or auxiliary piston 26 hits chamber 46 as the table is lowered.

The operationof the lifting device is as follows: Chamber 23, passage 20, chamber 35 and working chamber 52, including all connecting bores, are always completely filled with an incompressible medium usually hydraulic fluid. It must be ensured that the crosssection of auxiliary piston 26, and hence that part of chamber 35 transversed by it, and the annular free cross section of chamber 23, filled with fluid, are of equal size. When operating handle 76 presses operating pin 61 and hence valve lifter 39 downward, the hydraulic fluid can flow out of chamber 23, through radial bore 22, passage 20, bore 38, openings 45, annular groove 40, bore 41 and fluid openings 50 into chambers 35. This flow is produced by the fact that a resultant outwardly directed force acts on the side of piston 17 that faces the gas chamber, and said force is equal to the cross section of piston 17 minus the cross section of auxiliary piston 26 multiplied by the pressure of the gas in gas chamber 24. This force drives piston rod 14 outof housing 11 with simultaneouslifting of table top 75, whereupon (as described) hydraulic fluid flows from chamber 23 into chamber 35. The total volume of all chambers filled with hydraulic fluid does not change in the process. A

After operating pin 61 is released, valve lifter 39 returns to its upper, resting position under the pressure of spring 47, so that chamber 35 is again tightly sealed with respect to chamber 23. The process of lifting the table is thus ended, and table top 75 remains firmly in this new position.

To lower the table, pump piston rod 53 is moved up and down uniformly with an operating handle. During the downward stroke, the hydraulic fluid flows out of the part of working chamber 52 that is located below pump piston 56, through axial bores 58 in pump piston 56 into the section of working chamber 52 located above pump piston 56. As this movement progresses, check valves 59 allow free flow through axial bores 58. When pump piston rod 53 is moved upward, i.e., in a direction away from housing 11, the oil that is in the part of working chamber 52 that is above pump piston 56 is squeezed through bore 60 into passage 20 and thence through radial bore 22 into chamber 23. Simultaneously,the same volume of oil is drawn from chamber 35 through bore 64 in partition 30, annular passage 63 in partition 30, and bore 62 in pump housing 31 into the enlarged area of working chamber 52 beneath pump piston 56, since check valves 59 close off axial bores 58 during this motion of the pump piston rod 53 andhence the pump piston 56. Hence, a few strokes of the pump suffice to force piston 17 (and with it piston rod 14) against the pressure of the gas located in gas chamber 24, back into the housing, so that table top 71 is lowered uniformly.

In the embodiment according to FIG. 3, the construction of pump piston rod 53 is such that with short pump strokes only the hydraulic fluid is circulated, with simultaneous lowering of the table, since valve lifter 39 is not activated by pump piston rod 53'. A single push of pump piston rod 53 into housing 11 deep enough to displace valve lifter 39 pushes piston rod 14 out of housing 11 and thus lifts the table. I

FIG. 5 is a further especially simple embodiment of a lifting device according to the invention. In FIG. 5,

. those parts that are the same as those in the sample embodimentshown in FIG. 1 have the same key numbers,

so that reference will be made to those parts as they appear in FIG. 1. g Y

A slight difference can be seen in the fact that in the embodiment according to FIG. 5, bores 22 are not in centering ring 21 but in tube 18 near the centering ring,

which naturally can be done as well in the sample embodiments of FIGS. 1, 2 and 3. In this embodiment, tube 18 extends as far as plug 12, to which it is attached axially and radially by means of centering fitting 101. Tube 25 is closed off by cylindrical intermediate fitting 102 at the end shown as the upper end in the drawing, and is simultaneously attached radially and axially. lntermediate fitting 102 has a projection 103, against which tube 25 is pressed tightly. The intermediate fitting itself has a connection 104 whose outside diameter corresponds to the inside diameter of steel tube 18. This connection 104 has a groove on its outer circumference, into which steel tube 18 fits by means of a projecting ridge. This ridge 105 makes the intermediate fitting 102 radially and axially non-displaceable with respect to steel tube 18 and simultaneously makes it gasand liquid-tight, so that tube 25 is also completely immovable.

At its upper end, intermediate fitting 102 has another connection 106 whose outside diameter corresponds to the inside diameter of steel tube 18. This connection is provided with one or more axial bores 107 which connect chamber 108, which is enclosed at top and bottom by connections 104 and 106 and laterally by tube 18, with working chamber 109 located above intermediate fitting 102. Chamber 108 is connected with a diagonal bore 111 in intermediate fitting 102 with a chamber 110 that is bounded by the bottom of intermediate fitting 102, tube 25 and the bottom of auxiliary piston 26.

Pump piston rod 112 is mounted axially displaceably in working chamber 109 above intermediate fitting 102 which extends up to upper plug 12, said rod being mounted coaxially to housing 11 and working chamber 52, which in this case is bounded laterally by tube 18. This pump piston rod is constructed in similar fashion to that in FIG. 3. At one end it is fed through a lip seal in plug 12 and thus sealed gasand liquid-tight with respect to the outside; at the other end it passes through coaxial bore 114 in intermediate fitting 102; this bore 114 has no connection with chamber 108 or chamber 110. Pump piston rod 112 is mounted in bore 114, sealed tightly by seal 115. At the end that faces bore 114, pump piston rod 112 is partially hollowed out, so that even when pump piston rod 112 is fully driven into bore 114, there is still sufficient space to accomodate the gas that remains in bore 114, air for example.

In working chamber 109, pump piston 116 is fastened to pump piston rod 112, said piston sliding on the internal wall of steel tube 18 and carrying a seal 117 in a groove on its outside circumference. The pump piston has a large number of axial bores 118 arranged crosswise. As in the embodiments shown in FIGS. 1 and 3, a ring of sealing material 119 is mounted on pump piston rod 112 above these axial bores, with the travel of this ring on pump piston rod 112 being limited at the upper end by Seeger circlip 120. In its lower position, the ring of sealing material 119 rests firmly on axial bores 118, so that this ring serves as a check valve.

in one or two axial bores, 118, pins 121 are looselymounted, said pins being sufficiently long to lift the valve ring 119 off axial bores 118 when pump piston rod 112 is completely depressed. The maximum travel of pump piston rod 112 in the lifting device is limited by stop 122. On the other hand, the total travel of pump piston rod 112 and hence pump piston 116 is set so that when pump piston 116 is fully raised against the bottom of plug 12, pins 121 resting on connection 106 will not fall out of axial bores 118.

The working chamber 109 is connected in its upper area with passage 20 by means of bores 123 in tube 18. On the inside of plug 12 a concentric groove 124 is provided which is so deep that Seeger circlip slides freely into this groove 124 when pump piston 116 is brought to its upper, resting position, whereupon valve ring 120 rests firmly against plug 12 on the one hand and tight against axial bores 118 on the other.

The method of charging gas chamber 24 with gas is shown in FIG. 6, with intermediate fitting 102 shown enlarged. Gas chamber 24 is connected with bore 114 by means of bore 125 located in intermediate fitting 102. Through bore 125, gas is compressed into gas chamber 24 and hence chamber 28 in auxiliary piston 26, if pump piston rod 112 and plug 12 have not yet been raised. From bore 114, a threaded bore 126 leads to bore 125, into which threaded bolt 127 is screwed, having sealing plug 128 attached to its lower end. Before beginning the gas charging process, threaded bolt 127 and sealing plug 128 are screwed into threaded bore 126 just far enough so that bore 125 is not closed off but gas compressed through this bore into gas chamber 24 cannot leak back through threaded bore 126. After filling gas chamber 24 with compressed gas to the desired pressure, the threaded bolt is screwed down so that sealing plug 128 closes bore 125 tightly and prevents the gas from flowing back our of chamber 24. Then an incompressible medium, for example hydraulic fluid, is admitted to the lifting device, which is still open at the top; the fluid fills passage 20, chambers 23 and 110, chamber 108 and working chamber 109 on both sides of pump piston 116. After charging, plug 12 with lip seal 113 is pushed into housing 11 and fastened by rolling a seam and turning down the upper edge.

This embodiment works as follows: when pump piston rod 112 is completely depressed until it hits stop 122 on connection 106 of intermediate fitting 102, pin or pins 121 lift valve ring 119 off axial bores 118, so that the fluid is free to flow from lower chamber 23 through bore 22, passage 20, bore 123, the upper part of working chamber 109, axial bores 118, the lower part of working chamber 109, bores 107, chamber 108 and bores 111 to chamber 110. This opens the hydraulic blockage of the gas spring, so that the gas which is in gas chamber 24 and cavity 28 and is under pressure can expand and press piston 17 including piston rod 14 out of the lifting device as the table rises simultaneously.

To block the lifting device, i.e., to stop the lifting process, pump piston rod 112 is pulled all the way out of housing 11, so that valve ring 1 19 presses firmly against the under side of plug 12 and axial bores 118 are tightly sealed thereby.

To lower the table, pump piston rod 112 is moved back and forth between its upper, resting position and a lower one. The lower position is not so low that pin or pins 21 can forcibly raise valve ring 119 off axial bores 118.

During an upward stroke of pump piston rod 112 and hence pump piston 116, hydraulic fluid is forced from the upper part of working chamber 109 through bore 123, passage 20 and bore 22 into lower chamber 23. Simultaneously, hydraulic fluid is drawn through bore 111, chamber 108 and bores 107 from chamber 110 into the lower part of working chamber 109. When pump piston rod 112 is moved downward into the lifting device, valve ring 119 rests'agains't Seeger circlip 120, so that the fluid in the lower part of working that the valve part and the pump part are completely united. What is claimed is: r v b 1. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, compris ing a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tubewhich is'sealedwith respect to the outside, and with chambers provided which are filled withan incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other,

and wherein the chamber for the incompressible medium located on that side of the piston which faces the compressed gas is mounted in a separate tube in which an auxiliary piston is mounted by means of seals, said auxiliary piston serving to seal off the gas chamber from thischamber, and resting against the piston or fastened to it. e 2. A device according to claim 1, wherein auxiliary piston is'provided with a cavity that communicates with said gas chamber. I 3. A device according to claim 1 wherein saidtauxiliary piston is longer than the maximum travel of said piston.

4'. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fit ted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other,

and wherein said pump is mounted coaxially with said gas spring and a valve device in a common housing. j

5. In a hydraulically lockable lifting device particularly adapted for height adjustment of tables, comprising a gas spring, in which a'piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in-a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to'pump the incompressible medium from one chamber into the other,

and wherein an activating pin for said valve device and a pump piston rod for said pump are combined and brought together out of the housing.

6. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in whicha piston,'subject to the action of compressed gas and mounted on a piston rod, is litted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by .a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other; wherein said pump is made in the form of a plungerpiston pump; and wherein said plunger-piston pump comprises a pump piston provided with check valves and traveling in a working chamber, said piston being axially driven by means of a pump piston rod.,

7. A device according to claim 6 wherein each of the two parts of the working chamber separated by said pump piston is provided with a chamber. v

8. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside,

and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein said pump serves simultaneously as a valve, said pump comprising a piston having a valve ring thereabout, a cylinder in which said piston moves, and axial bores extending through said cylinder for mating with said valve ring upon a predetermined travel distance of said piston. 9. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in whicha piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein said pump serves simultaneously as a valve, wherein operating pins are provided for forcible opening of a check valve when said pump piston is at one terminus of its travel. 

1. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein the chamber for the incompressible medium located on that side of the piston which faces the compressed gas is mounted in a separate tube in which an auxiliary piston is mounted by means of seals, said auxiliary piston serving to seal off the gas chamber from this chamber, and resting against the piston or fastened to it.
 2. A device according to claim 1, wherein auxiliary piston is provided with a cavity that communicates with said gas chamber.
 3. A device according to claim 1 wherein said auxiliary piston is longer than the maximum travel of said piston.
 4. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein said pump is mounted coaxially with said gas spring and a valve device in a common housing.
 5. In a hydraulically lockable lifting device particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein an activating pin for said valve device and a pump piston rod for said pump are combined and brought together out of the housing.
 6. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of The piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other; wherein said pump is made in the form of a plunger-piston pump; and wherein said plunger-piston pump comprises a pump piston provided with check valves and traveling in a working chamber, said piston being axially driven by means of a pump piston rod.
 7. A device according to claim 6, wherein each of the two parts of the working chamber separated by said pump piston is provided with a chamber.
 8. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein said pump serves simultaneously as a valve, said pump comprising a piston having a valve ring thereabout, a cylinder in which said piston moves, and axial bores extending through said cylinder for mating with said valve ring upon a predetermined travel distance of said piston.
 9. In a hydraulically lockable lifting device, particularly adapted for height adjustment of tables, comprising a gas spring, in which a piston, subject to the action of compressed gas and mounted on a piston rod, is fitted in a tube which is sealed with respect to the outside, and with chambers provided which are filled with an incompressible medium and are located on both sides of the piston, said chambers linked by a closable connecting passage, the improvement wherein a pump is provided to pump the incompressible medium from one chamber into the other, and wherein said pump serves simultaneously as a valve, wherein operating pins are provided for forcible opening of a check valve when said pump piston is at one terminus of its travel. 